This continuing Program Project Grant (PPG) proposal comprises three Projects and two Cores. The focus of the projects is identification of molecular signals that initiate cardiac arrhythmias and sudden cardiac death (SCO). A central hypothesis is that perturbation of intracellular Ca signaling can lead to triggers of cardiac arrhythmias. We propose that perturbations of intracellular calcium homeostasis, primarily due to defective regulation or function of ion channels including potassium, sodium, calcium channels as well as the ryanodine receptor/calcium release channel on the cardiac sarcoplasmic reticulum, initiate triggers of fatal cardiac arrhythmias. We further propose that in humans variant forms of adrenergic receptors predispose subsets of individuals to increased risk of fatal cardiac arrhythmias via effects on intracellular calcium signaling. Important advances in our understanding of the molecular basis of sudden cardiac death due to diastolic leak of Ca from the sarcoplasmic reticulum via the cardiac ryanodine receptor (RyR2) have resulted from this project. These advances have lead directly to potential novel therapy for Sudden Cardiac Death (Fig. 1). In addition, this project has contributed to the discovery of a novel mechanism for regulating the IKS potassium channel (KCNQ1/KCNE1) via sympathetic nervous system stimulation. We showed that a defect in this mechanism is responsible for sudden cardiac death in patients. The three projects are integrally linked as follows: 1) Project 1, PI - Andrew R. Marks, M.D., Chair of Physiology and Cellular Biophysics, Columbia University, Director, Center for Molecular Cardiology, Columbia University, has three aims focused on the functional characterization of the effects of adrenergic modulation on the cardiac ryanodine receptor/calcium release channel. 2) Project 2, PI - Robert S. Kass, Ph.D., Chair of Pharmacology, Columbia University, has two aims focused on characterization of contributions of sodium channel sustained activity to calcium release-triggered sudden cardiac death (SCO) and on modulation of sarcolemmal ion channels by SR calcium release;3) Project 3, PI -- PI W. J. Lederer, M.D., Ph.D., Director Medical Biotechnology Center, University of Maryland, has three aims focused on determining the effects of ion channel mutations and adrenergic modulation on calcium signaling in cardiomyocytes. Two Cores are proposed as follows: 1) Administrative (A.R. Marks, PI);2) Animal Model Core. (J. D'Armiento, PI - This Core will generate and provide genetic animal models of ion channel diseases to each of the four projects).